So-called core-routers or core-switches applied for IP (Internet Protocol) networks, generally adopt redundant architecture. As shown in FIG. 90, the core-switch 1004a, and 1004b are disposed in duplicated manner. The core-switch 1004a and 1004b are connected through transmission line 1009 to share routing information. If the one of core-switches failed another core switch takes over job of the downed core-switch.
Edge-switches 1005a, 1005b, 1005c, and 1005d are connected to both core-switches. Each edge-switch is connected to both of core-switches, 1004a and 1004b. When two core-switches are under operation, traffic load is shared by two core-switches. When one of the core-switches failed, the surviving core-switch takes over entire traffic load.
VRRP (Virtual Router Redundancy Protocol) and MSTP (Multiple Spanning Tree Protocol) defined in IEEE802.1S are cited as redundant control protocol as shown above.
Furthermore, client stations 1006 are connected to the edge-switch 1005. A back-bone network 1008 is connected to the core-switch 1004a and 1004b. 